. 179. OF COMPOUND MINERALS. 247 



The above mentioned twin-crystals occur in octahedral 

 Iron-ore, in octahedral and dodecahedral Corundum, &c. 

 Vol. II. Fig. 156. Its crystallographic sign is O,{}. 



Instead of the octahedrons, we may substitute two hexa- 

 hedrons, without changing any thing in the law of compo- 

 sition ; the resulting twin-crystal will have its face of 

 composition parallel to a face of the octahedron, and that 

 rhombohedral axis, which is common to the individuals, 

 for its axis of revolution. The re-entering angles are 

 = 250 31' 44" = 360 109 28' 16". 



Regular compositions of this kind are found in hexahe- 

 dral Lead-glance, also such twin-crystals as result from com- 

 binations of the hexahedron and the octahedron, or other 

 forms of the species. The crystallographic signs of the 

 first are H, {} ; of the second H.O, {?}. 



The dodecahedron, if substituted instead of the octahe- 

 dron, produces similar twin-crvstals, which are found in 

 octahedral Diamond, dodecahedral Garnet-blende, &c. 

 Vol. II. Fig. 163. Their designation is D,{}. , 



If we join two rhombohedrons in a transverse position, 

 the principal axis being the axis of revolution, the twin- 

 crystal produced will be of the same kind as the preceding 

 ones, which becomes evident, if we imagine a rhombohe- 

 dron to take the place of the above mentioned hexahedron. 

 The face of composition, which is perpendicular to the 

 rhombohedral axis, and, as such, analogous to that of the 

 octahedron, is parallel to H 03. We may now substi- 

 tute any finite rhombohedral form, for instance (P) 3 , or 

 any combination of rhombohedrons, pyramids, and prisms, 

 instead of the rhombohedron ; and the result will yield 

 forms of the same kind, of which many examples are 

 found in rhombohedral Lime-haloide. Vol. II. Fig. 129. 

 The crystallographic sign of this regular composition is 



(P),{R_.}. 



Let every thing be as before, only the axis of revolution 

 not parallel to an axis of crystallisation. 



Twin-crystals of this kind are produced, if two rhombo- 



